Method and apparatus for determining the average size of apertures in an apertured member

ABSTRACT

The average aperture width in a small area of an apertured member, such as a shadow mask for a cathode-ray tube, is determined by passing a beam of substantially monochromatic light through an area of the member to form an interference pattern, detecting the intensities of at least two light fringes of the interference pattern, generating electrical signals which are representative of the detected intensities and then deriving the average width of apertures in the lit area of the member from the generated signals.

BACKGROUND OF THE INVENTION

This invention relates to a novel method and apparatus for measuring theaverage size of a group of apertures in a member having an array ofsimilar-sized apertures therein. The invention is especially applicableto measuring the apertures of an apertured mask for a cathode-ray tube.

One type of color television picture tube employs a slit-type aperturedmask. Such as mask is comprised of a metal sheet, about 4 to 8 milsthick (about 0.10 to 0.20 mm.), having an array of slits which are about3 to 10 mils wide (about 0.075 to 0.25 mm.) whose center lines aresubstantially uniformly spaced about 20 to 80 mils (about 0.5 to 2.0mm.) apart in parallel rows or columns. The slits may be of uniformwidth, or the widths may gradually become smaller from center to edge.The slits may be uniform, graduated, or random length in the rows, andare generally about 30 to 60 mils long. At least for quality-controlpurposes, it is desirable during manufacturing to check the widths ofthe slits from area to area on a single mask and also to check thewidths of the slits from mask to mask.

It is known that projecting a beam of substantially monochromatic lightfrom one side through an array of parallel slit apertures ofsubstantially uniform widths and spacings produces combined interferenceand diffraction patterns in a plane spaced from the opposite side of theaperture array. The interference pattern is comprised of alternate lightand dark bands or fringes. The diffraction pattern is an envelopedefined by the peak intensities of the light fringes of the interferencepattern. The envelope is also comprised of alternate light and darkbands including a central maximum and uniformly-spaced side maxima(light bands) separated by minima (dark bands). The physical dimensionsand spacings of the interference bands and diffraction bands areproportionately related to the average widths and spacings of theapertures which produce them. By prior methods, the average aperturewidth was derived by measuring one or more of these distances and thencalculating the average width. Such prior methods are slow, and theresults are not as precise as are desired for quality control duringmask manufacturing. Such prior methods do not lend themselves toautomation by modern electronic techniques.

SUMMARY OF THE INVENTION

The novel method is based on the discovery that the average aperturewidth of an apertured member bears a specific relationship to theintensities of the light fringes constituting the combined interferenceand diffraction patterns produced by the member. This relationship canbe closely approximated algebraically over a range of interest in amanner that allows the average aperture width to be calculated rapidlyand accurately by electronic techniques.

The novel method comprises projecting a beam of substantiallymonochromatic light through a plurality of apertures in a relativelysmall area of an apertured member to produce combined interference anddiffraction patterns. Then, the intensities of two light fringes of theinterference pattern, preferably in the central maximum of thediffraction pattern, are sensed and generate electrical signals whichare a function of the intensities of the fringes. An electrical signalis then derived from the ratio of the generated electrical signals,which derived signal is directly representative of the average widths ofthe apertures within the beam. A novel apparatus for determining averageaperture width includes means for projecting a beam of monochromaticlight through a plurality of apertures in an apertured member to producecombined interference and diffraction patterns, and means responsive tosaid pattern for generating a signal therefrom which is representativeof said average width.

In one form of the invention, a low-power laser beam or other source ofmonochromatic light in a fixed position projects a beam through theapertured member upon two photocells, which are in fixed positions withrespect to one another and fixed distances from the member. In apreferred form, the beam is so refracted as to focus the beam upon thephotocells, each of which generates an electrical signal in response tothe incident light. The member may be stationary or moving linearly whenthe reading is made since the pattern remains stationary irrespective oftranslational motion of the work piece. The light beam may vary inintensity or brightness, due, for example, to variations in linevoltage. However, the ratio of the intensities of one measured fringe tothe other remains constant. With a simple electronic processing circuit,the average width of the apertures within the beam is derived from thegenerated electrical signals. The derived signal may be used to actuatesome automatic process or may be employed to actuate a display of theaperture width.

Only the average width of the apertures is a variable in any particularapparatus set-up. The center lines of the rows of the apertures withinthe beam are substantially uniformly spaced. The wavelengths of light ofthe beam are fixed by the choice of the source. The spacing of the workpiece to the pattern plane is fixed by the design and adjustment of theequipment. A variation in the average slit width results in a variationin the ratio of the intensities of the interference fringes, whichfringes are fixed in position in the plane of the photocells withrespect to one another and with respect to the center line of the lightbeam by the adjustment of the equipment. If, in the apertured member,the center-to-center spacing between adjacent apertures variessubstantially, the position of the fringes will shift and cause anapparent change in aperture width. Additional detectors can be used tosense the change in fringe position and can compensate theaperture-width output as well as being used to indicate the extent ofthe shift. This allows measurement of masks when the aperture spacing isnot constant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially-schematic fragmentary front view of a novelapparatus for carrying out the novel method.

FIG. 2 is a fragmentary plan view of a mask having an aligned array ofslit apertures therein.

FIG. 3 is a diagram of a circuit used with the apparatus of FIG. 1.

FIG. 4 is a schematic diagram used to explain some of the opticaleffects used to carry out the method of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a lower frame 21 and an upper frame 23 mounted in fixedrelation to one another by means that are not shown. The bottom side ofthe lower frame 21 carries a laser-mounting bracket 25 on which ismounted a helium-neon laser tube 27 which is held in two guides 29 and31 by two springs 33. A ten-power converging lens in a lens holder 35 ismounted on the output side of the laser tube 27, so that a light beam 28from the laser tube 27 may be projected upward through the lens in amanner that avoids destructive effects to persons working in the area.Any monochromatic-light source may be used in place of the particularlaser tube 27. A low-power helium-neon laser is preferred because of itsrelatively low initial cost, low cost of operation and relatively highsafety. The lens may be of any convenient size, power and opticalcharacteristic capable of producing a light spot of the desired size inthe plane of the apertured mask to be measured and also a focused spotin the plane of the detector 61 described below.

The lower frame 21 has an opening 26 therein to permit the light beam 28from the laser tube 27 to pass upward therethrough, and carries ashutter 30 for intercepting or passing the beam 28 as desired. The topside of the lower frame 21 supports a mask carrier 37 and means forguiding the movement thereof in a generally horizontal plane, normal tothe path of light beam. The carrier 37 includes a removable insert 39having an open portion adapted to shape and size to receive an aperturedmask 41 to be measured. The carrier 37 may be moved in one direction(designated the y direction) on two y guide rods 43 on two supports 45.The supports 45 may be moved in the other direction (designated the xdirection) on two x guide rods 47 supported on the lower frame 21 in amanner not shown. The arrangement of x and y guide rods 43 and 47permits the apertured mask 41 in the carrier 37 to be translated rapidlythrough the beam 28 to measure different areas on the mask.

The top portion of the upper frame 23 supports a light-tight box (notshown) in a position roughly centered over the laser tube 27 and acrossplate 51. The upper frame 23 has an opening 49 therein to permitthe light beam 28 from the laser tube 27 to pass upward therethrough. Avertical mounting post 53 supported by the crossplate 51 carries a firstadjustable support 55 and a plane mirror 57 thereon. A second adjustablesupport 59 carrying a fringe detector 61 thereon is supported on thecrossplate 51. In operation, the supports 55 and 59 are adjusted for themirror 57 to receive and reflect light from the laser tube 27 to thefringe detector 61.

The fringe detector 61 comprises two photocells P1 and P2 in a singlecontainer a known distance apart and is used to detect simultaneouslytwo different fringes of an interference pattern. Of course, photocellsin separate containers may be used. More than two photocells may beused. The outputs of the fringe detector 61 are fed to a circuit system,which is described in greater detail with respect to FIG. 3, comprisingtwo current-to-voltage converters C1 and C2, a settable reference-signalgenerator G1, and signal processor S1, and an "add-k" circuit A1. Thecircuit system derives the desired width dimension and then displays itnumerically on a display D1.

FIG. 2 shows a fragment of an apertured mask 41 having a plurality ofslit apertures 42 arranged lengthwise in columns and spaced apart inuniform center-to-center spacing a with respect to the next adjacentcolumn of apertures. The apertures ideally should have identical widthsb. In producing the apertured mask 41, the center-to-center aperturespacing a is, by the nature of the manufacturing process, fixed for agiven apertured mask. There is a negligible variation of thecenter-to-center spacing a from one apertured mask to another. However,the apertures are produced by a process wherein the slit widths b mayvary substantially in the same mask and from mask to mask. Therefore, itis important to be able to ascertain the average width b of a pluralityof apertures 42 over small areas in a given apertured mask.

The present invention takes advantage of the fact that when amonochromatic light beam is projected through an array of apertures, aninterference pattern is formed wherein the ratio of the intensities oftwo fringes of the pattern, preferably adjacent fringes in the centralmaximum, is a function of the average aperture width of the areailluminated. It can be shown that the average width b is defined by thepolynomial equation:

    b = α + βR + γ R.sup.2 + . . . + δR.sup.n (1)

where R is ratio of the intensities. A good approximation of thisrelationship is

    b = k + V.sub.y R.sup.m                                    (2)

where k, V_(y) and m are preselected values that are determinedempirically for each type of mask. These values are determined for eachmask type. Such determinations are within the abilities of one skilledin this art. By type of mask is meant a mask having distinctive aperturesize range, aperture spacings, or other characteristics which have asubstantial effect on the values of k, V_(y) and m.

In the circuit system shown in FIG. 3, the two outputs of photocells P1and P2 representing the intensities of two fringes of the interferencepattern are used to produce signals V_(x) and V_(z) in thecurrent-to-voltage converters C1 and C2 respectively. The circuit thenderives the average aperture width b according to the relationship b =k + V_(y) (V_(x) /V_(z))^(m) where k, V_(y), and m are present valuesfor each type of mask or other apertured member. The output is thenapplied to a suitable display D1, which may be a commercially-availabledigital panel display apparatus. The display D1 includes ananalog-to-digital converter and a digital display device for displayingthe magnitude of the analog signal applied as an input thereto.

The converters C1 and C2 are substantially identical, and therefore adescription of only one will be provided. The same numbers with andwithout the primes refer to similar parts in the converters C1 and C2.Converter C1 is comprised of an operational amplifier 56 and a filteringcapacitor 58 in parallel with serially-connected resistances 60 and 62connected between the output and the inverting input of the amplifier56. The noninverting input of amplifier 56 is connected to a point ofreference potential such as ground. The values of the resistances 60 and62 determine the gain of the operation amplifier 56. The resistance 62is variable for adjusting the output voltage on the output lead 69within a desired range suitable for use with the signal processor S1.The capacitor 58 serves as a low pass filter for filtering outinstantaneous changes in intensity in the light source. The outputsignal V_(x) on the output lead 69 of converter C1 is a signal whosevoltage amplitude represents the intensity of the light received by thefirst photocell P1. In a similar manner, the voltage amplitude of thesignal V_(z) on output lead 71 from converter C2 represents theintensity of the light received by the second photocell P2.

The output leads 69 and 71 are connected to the input of the log ratiodevice 64 which forms a part of the signal processor S1. The log ratiodevice 64 produces a signal whose voltage magnitude is representative ofthe logarithm of the ratio of the amplitude of signal V_(x) with respectto the amplitude of the signal V_(z). The output of the log ratio device64 is applied through a lead 66 to a serially-connected resistance 68and variable resistance 70 and then to a summation device 72 through afirst wiper arm 73. The values of the resistances 68 and 70 determinethe value of the exponent m, the power to which the ratio V_(x) /V_(z)is raised.

A reference-signal generator G1 includes a source of a reference voltage74 whose output is connected to a reference potential such as ground ata terminal 76 through voltage-dividing resistors 78, 80 and 82. A secondwiper arm 86 on the resistance 80 is connected to the output lead 84 ofthe generator G1. The setting of the second wiper arm 86 determines thevalue of the generated signal V_(y) appearing on the lead 84.

The lead 84 is connected to the signal-processor summation device 72through a logarithm device 88 which produces the logarithm of the valueof the amplitude of signal V_(y). The summation device 72 produces asignal representing the sum of the logarithm of signal V_(y) and thelogarithm of the ratio of signals V_(x) /V_(z) raised to the m power, orlog V_(y) (V_(x) /V_(z))^(m). This signal is applied as an input to anantilog device 90 which provides a signal whose amplitude represents theantilog of the signal applied to the input thereof. The output signal ofthe antilog device 90 is applied along lead 92 to an adder device A1,which adds the value k to the signal-processor output. The adder deviceA1 is comprised of a voltage source 95 and a variable resistor 97 inparallel. A third wiper arm 96 (whose position determines the value ofk) of the variable resistor 97 is connected to a suitableanalog-to-digital converter and digital display device D1.

The log ratio device 64, the antilog device 90, the logarithm device 88,the summation device 72 and the reference voltage source 74 arecommercially-available devices. Such devices are manufactured in asingle multifunctional module; for example, model number 433Jmanufactured by Analog Devices, Inc. of Norwood, Mass. A description ofthat device is provided in a catalog C125-10 dated May 1972 andpublished by Analog Devices, Inc.

In setting up the apparatus for operation, the height of the platform 55above the crossplate 51 is set to give the desired fringe spacing at thedetector 61. In one example, the light path from the holder 39 to thedetector 61 is about 60 inches and the centers of the photocells P1 andP2 are about 0.054 inch apart. The outputs of the converters C1 and C2are adjusted so as to be substantially equal with equal amounts of lightincident on the photocells P1 and P2. This can be accomplished bysuitable display means, such as the display D1, temporarily coupleddirectly to the leads 69 and 71 by means not shown. The circuit for thesignal processor 54 described above is responsive to voltage inputsalong the leads 69 and 71 having a value in the range of 0 to 10 volts.The outputs of the converters C1 and C2 are adjusted by adjusting theresistances 62 and 62' respectively. With no mask 41 in place and equalamounts of light on the photocells P1 and P2, the signals V_(x) andV_(z) are matched. Then, a mask 41 is placed in the mask holder 37 and abeam 28 is passed through the mask. The lens in the holder 35 isadjusted to focus the beam on the detector 61, and the detector 61 ispositioned in the focused beam so that the desired fringes are detectedand so that the value of V_(z) is greater than the value of V_(x).

Three standard aperture masks of the same type whose aperture widths bare maximum, bogie, and minimum respectively (as predetermined byprecise measurements in a measuring microscope or the like) areprovided. The mask with maximum aperture size is first inserted in themeasuring light beam 28. The third wiper arm 96 is now positioned sothat a correct reading is shown on the display D1. Next the mask withminimum aperture size is inserted into the beam 28. The second wiper arm86 in the settable reference signal generator G1 is positioned so that acorrect reading of the hole size appears on the display D1. Next, themask with the bogie aperture size is inserted in the light beam 28 andthe first wiper arm 73 of the resistor 70 is positioned so that acorrect reading appears on the display D1. With the first, second andthird wiper arms 73, 86 and 96 so positioned, which determines thevalues of m, V_(y) and k respectively, the procedure is repeated toposition these wiper arms more precisely so that a proper readingappears on the display for each of the three standard masks inserted inthe light beam 28. The positions of the three wiper arms 96, 86, and 73are noted and these values can be used whenever masks of the same typeare to be measured. A similar setting-up procedure is employed for eachtype of mask.

To operate the apperatus of FIGS. 1 and 3, a mask 41 with slit-shapedapertures therein is placed in the holder 39. The mask may be a flat,etched sheet prior to being formed into the domed shape for use in acolor television picture tube. However, the apparatus may also be usedwith a formed mask. The laser tube 27 is turned on so that the beam 28is projected upward and a proper positioning of the three wiper arms 96,86, and 73 is made in accordance with the positions noted for that typeof mask. The mask holder 39 is slid into a desired position on the x andy guides 43 and 47, and the average aperture width for the areailluminated by the beam appears on the display D1. The mask holder 39may be moved to different positions to measure the average widths ofapertures at different areas of the mask. Then, the mask may be removedfrom the holder, a different mask of the same type is inserted thereinand the measurement process repeated.

FIG. 4 illustrates in simplified form what happens in the optical systemas the beam 28 passes from the laser tube 27 through the aperturedmember 41 to the detector 61. The laser tube 27, a 5-mw HeNe laser, isfitted with a lens system that expands the laser beam and then focusesthe expanded beam in the plane 109 of the detector 61. By changing thepower of the lens system, the beam size can be changed in order tochange the number of mask apertures in the measurement. In one system, a10 power magnification lens produces a beam that is about 1/2 inch indiameter where it passes through the mask 41. The intensity across thebeam 28 has nearly a Gaussian distribution; consequently the aperturesnear the center of the beam transmit more light than those near theedges of the beam and have a stronger influence on the measurements. Themask 41 is placed so that the beam impinges on the mask 41 perpendicularto its surface. As the beam strikes the mask 41, diffraction occurs andfringes 106 of an interference pattern can be observed in the detectorplane 109, which is placed at a convenient distance from the aperturedmask 41. The greater the distance, the greater the spacing of thefringes of the interference pattern. The fringes 106 are substantiallyuniformly spaced a distance d.sub. i apart. The detector 61 ispositioned to receive two adjacent light fringes 106a and 106b on thetwo photocells P1 and P2 respectively. In FIG. 4, the brightness offringes is indicated by the horizontal distance of the curve 106 fromthe detector plane 109. With the light source focused in the plane 109of the detector 61, the pattern appears stationary even when the mask 41is moved linearly. Rotation of the mask will cause the pattern to rotatein the plane 109, but it will remain centered on the same axis.

Also in FIG. 4, there is shown an envelope 107 defined by the peaks ofthe fringes 106, which defines a diffraction pattern. The diffractionpattern comprises a central maximum and side maxima. The distance d₁between the peaks of the fringes 106 is an inverse function of thedistance between rows of apertures in the mask 41. The width d_(d) of aside maximum equals the half width d_(o) of the central maximum of thediffraction pattern, and are inverse functions of the aperture width.The relative height or intensities of the various fringes 106 of theinterference pattern are determined by aperture width.

The novel device provides fast, accurate and reliable means of measuringthe sizes of shadow-mask apertures. The device was developed primarilyfor measurement of slit widths in flat, slit-type shadow masks. It hasshown a capability for measuring webs (the distance between the ends oftwo slit apertures in a row) and can be extended to measure apertures informed masks and masks having round apertures as well as mask masters.Some advantages of this method over previously employed methods(transmission and microscope) are

1. improved accuracy,

2. faster speed of measurement, and

3. improved ability to average a large number of apertures or to measureindividual apertures with the same operating speed.

I claim:
 1. A method for determining the average width of the aperturesin a limited portion of a regular array of similar apertures in anapertured member, said method comprisinga. projecting a beam ofsubstantially monochromatic light through a plurality of apertures insaid limited portion of said member, thereby producing combined lightinterference and diffraction patterns containing the desired widthinformation, b. detecting the intensities of at least two interferencefringes of said patterns, c. generating electrical signals which arerepresentative of said detected intensities, and then d. deriving fromsaid generated electrical signals the average width of said aperturesthrough which the beam has passed.
 2. The method defined in claim 1wherein said apertures are substantially rectangular slits arrangedlengthwise in rows.
 3. The method defined in claim 2 wherein saidapertures are about 3 to 10 mils wide and are located on center lineswhich are substantially uniformly spaced about 20 to 80 mils apart. 4.The method defined in claim 2 wherein said detected fringes include thetwo adjacent fringes on one side of the central maximum of saiddiffraction pattern.
 5. The method defined in claim 1 wherein saidderiving step (d) includes producing from said generated signals a ratiosignal which is representative of the average width of said apertures,and then converting said ratio signal into an average width value.
 6. Amethod for determining the average width of the apertures in a limitedportion of a regular array of similar apertures in an apertured mask fora cathode-ray tube, said mask having two opposite major surfaces, saidmethod comprisinga. projecting a beam of substantially monochromaticlight from a source spaced from one of said major surfaces through aplurality of apertures in adjacent rows of said limited portion of saidmask, b. focusing said beam in a plane spaced from the other of saidmajor surfaces of said mask, thereby producing combined lightinterference and diffraction patterns in said plane, c. detecting theintensities of at least two interference fringes in the central maximumof said diffraction pattern, the ratio of said intensities being afunction of the average width of said plurality of apertures, d.producing from said detected intensities an electrical signal which isrepresentstive of the ratio of said intensities, e. processing saidproduced electrical signal to provide an output electrical signal of avalue which is directly representative of said average width of saidplurality of apertures.
 7. The method defined in claim 6 including thefurther step of displaying the average width manifested by said outputelectrical signal.
 8. An apparatus for determining the average aperturewidth of a plurality of apertures in a cathode-ray tube apertured maskcomprisingmeans for projecting a beam of substantially monochromaticlight through said plurality of apertures in said portion of said maskto produce a combined light interference and diffraction patterncontaining aperture width information, means responsive to said patternfor providing at least two signals representing the intensities of atleast two fringes of said pattern, and means responsive to said twosignals for generating an electrical output signal therefromrepresenting the average width of said plurality of apertures.
 9. Theapparatus defined in claim 8 wherein said projecting means includesmeans for producing a laser beam along a prescribed path, means forfocusing said beam upon said responsive means, and means for holdingsaid apertured mask in said beam with the major surfaces thereofsubstantially normal to the path of said beam.
 10. The apparatus definedin claim 9 wherein said mask-holding means includes also means fortranslating said mask in a plane that is substantially normal to thepath of said beam.